Conspiracies in literature

I love conspiracy theories, I love intrigue and double-crossing and I love secret societies - the older, weirder and more sinister the better. Robert Rankin is a great one for this, and I quite liked Iain Banks's The Business, about a clandestine company that dates from the days of the Roman republic. Then you've got Lovecraft's various cults and secret societies, and I have to say I enjoyed this aspect of The da Vinci Code (it's just a shame about the one-dimensional characterisation, clunky prose, gratuitous technical descriptions, appallingly cliched posh-bumbling-English-twit-who-turns-out-to-be-the-baddie and cop-out ending).

Can anyone point me towards some more half-decent fiction of this sort?

Robert Anton Wilson's Illuminatus trilogy is probably the grandaddy of a lot of this stuff. Very tied to the hippy era it was written in tho.

Yeah, I read that - wait a sec, no I didn't, I read the Schrodinger' Cat trilogy (by R.A.W. and some other guy also named Robert, I think). Which I found frequently enjoyable, but far too all-over-the-place and, well, pretentious not to be quite annoying. Is Illuminatus! in the same sort of vein? (if you've read SC or can compare I! to my description of it...)

Edit: Cryptonomicon is fantastic - read it a few years ago and I think it's probably one of my favourite books.

the writing about conspiracy theories, fourierism, and all that in The Arcades Project sort of trumps everything i've read in novels. with the exception of pynchon who just approaches it all differently anyway...

"I never read/watched The Da Vinci Code but I did read The Holy Blood and the Holy Grail when it came out and from what I understand of the former they do seem awfully similar in content."

Well, they deal with the same legend don't they? HB&HG purports to have discovered the truth (their work since discredited) and DVC gives an entirely fictional (and admittedly so) treatment of that truth. So does Foucault's Pendulum for that matter sort of.